Ever-increasing data rates and bandwidth are central to the continued evolution of wireless communication networks. Current cellular access systems operate at radio frequencies where the path loss between User Equipment (UE) and a base station is sufficiently low that a static antenna configuration can adequately cover a complete cell or sector.
Future systems will require much more data to be communicated over the air interface. The only cost-effective way to accomplish this is to employ higher radio frequencies, which have sufficient bandwidth for high bit rate communication. To achieve sufficiently low path loss and spatial isolation, large antenna arrays will be used in the uplink (i.e., communications from UEs to the base station). These arrays have a large number of antenna ports, where data signals are communicated via each port. An antenna combining device will transform antenna port information to UE information streams, also referred to as layers, by linearly combining signals from different antenna ports. As used herein, a “layer” or information “stream” may refer to data flows from different UEs to the base station, several data flows from a single UE (e.g., MIMO communications), or a mixture of the two.
The antenna combining device will have the task to transform information from N antenna ports to L information layers. To enable good spatial selectivity, the number L must be significantly smaller than N. The antenna combining device attempts to achieve orthogonality between information flows from the different layers. This orthogonalization is achieved by utilizing spatial selectivity of the array and is typically called beam forming. The combining must be dynamic to adapt to changes in the environment over time.
Several known methods exist to implement such an antenna combining device. One approach relies on measured radio paths between all possible transmit and receive antenna pairs. Based on this information, different algorithms exist to implement antenna combining, depending on how the combining weights for the antenna ports are selected. Maximum Ratio Combining (MRC) effectively matches the receiver to the composite propagation channel of a given layer, and thereby maximizes the received power at the receiver. This is achieved by applying the conjugate of the spatial channel response, which is computationally simple but suboptimal in the presence of interference. Interference Rejection Combining (IRC) optimizes the Signal to Interference and Noise Ratio (SINR) of a layer considering the presence of other layer signals, steering both lobes and nulls in the array directivity diagram in appropriate directions. Performance is significantly increased, but an inversion of the signal covariance matrix is required, which has a high complexity for large antenna arrays.
IRC combining is optimal for extracting individual layer signals. However, this requires fully parallel implementation of all analog and digital circuits between the antenna elements and the combining circuits, which has a negative impact on cost, power consumption, and the dimensioning of interfaces. It is known to physically separate base station circuits, with circuits closest to the analog radio functionality having lower computational capacity than centrally-located baseband processing resources, which may service multiple radio devices. Due to the computational complexity of IRC combining—such as matrix inversion when using a large number of antenna ports—the processing should be performed by baseband computing resources. However, this requires large amounts of data to be transferred from the antennas to the baseband processors. Alternatively, MRC combining, which is not as computationally demanding, may be implemented in the radio receivers closer to the antennas. However, when signals from several UEs or layers are simultaneously received by the antenna array, the receiver performance of MRC combining is inferior to IRC solutions.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.